Stimulus Compounding in Interval Timing: The Modality-Duration Relationship of the Anchor Durations Results in Qualitatively Different Response Patterns to the Compound Cue

摘要

We have previously demonstrated that rats trained on a two-duration peak procedure in which 2 different modal signals (i.e., tone and houselight) predicted probabilistic reinforcement availability at 2 different times (10s and 20s), would respond in a scalar manner at a time between the trained durations in response to the simultaneous compound cue (tone + houselight). The present experiments evaluated whether this scalar response pattern would remain with greater relative separation between the anchor durations. Results revealed an effect of the modality-duration relationship, such that scalar responding was seen on compound trials in rats trained that the auditory stimulus signaled the shorter duration, while the visual stimulus signaled the longer duration, but not in the reverse condition. In those rats showing scalar responding on compound trials, post-hoc analyses demonstrated that the peak time of compound responding was most accurately predicted by the reinforcement probability weighted average of anchor peak times. In contrast, rats trained that the visual stimulus signaled the shorter duration, while the auditory stimulus signaled the longer duration, responded in a highly rightward skewed manner. In these subjects, initiation of responding to the compound stimulus appeared to be controlled by the visual stimulus only, while response terminations reflected control by both modal stimuli. These latter data provide evidence of separate determinants of response initiation and termination.